Laser diodes are used to optically pump optical fiber (gain fiber). The gain fiber can either be regular fiber in a Raman amplification scheme or rare-earth doped fiber, which has been spliced into the optical link to enable amplification of light signals. In common commercial products, 980 nanometer (nm) or 1480 nm diode lasers are used to optically pump erbium-doped fiber amplifiers operating or amplifying typically in a spectral range around 1550 nm.
In these diode pump-gain fiber systems, it is important to minimize changes in the amplifier characteristics due to changes in the pump wavelength or power. This is especially true in wavelength division multiplexing (WDM) systems or dense wavelength division multiplexing (DWDM) systems comprising many, spectrally closely-spaced channels. For example, mode hopping in the pump can cause changes in the gain spectrum of the amplifier. These changes result in preferential amplification of channels relative to other channels in the DWDM system.
One solution to controlling noise and wavelength shift due to environment temperature or power changes in the pumps uses fiber-grating stabilization. The Bragg grating has the effect of stabilizing the output spectrum from the laser pump or, more specifically, the grating stabilizes the pump against temporal power and frequency fluctuations. Further, in one suggested implementation, the grating is selected, spaced from the laser module, and tuned relative to the laser""s exit facet reflectivity so that the spectrum of the emission is broadened relative to that of a solitary laser.
To further stabilize pump lasers, polarization control is many times useful. The light emitted from the output facet of the diode lasers is typically highly polarized. The polarization of the light propagating through regular, non-polarization maintaining fiber, however, can change its orientation due to fiber birefringence, fiber twisting, bending, temperature shifts, and other stresses. Any fluctuation in the polarization of the light returning to the optical device from the grating effectively changes the feedback power ratio, because the laser is insensitive to any reflected light that has a polarization orthogonal to that of the emitted light. For example, if all of the reflected light has its polarization rotated by 90 degrees, the fiber Bragg grating is effectively removed from the system from the laser""s perspective.
In applications where polarization control is required between the laser diode and the grating, polarization-maintaining (PM) fiber is used for the fiber pigtail, with the grating being written into the PM fiber.
There are many different approaches to selecting facet and fiber grating reflectivities. Generally, the optimization seeks to maximize the pump module output, while minimizing temporal power fluctuations.
One approach suggests the use of an antireflected-coated laser diode output or exit facet. This yields a laser diode exit facet that has very low reflectivity, typically on the order of less than 0.5% power reflectivity. The grating reflectivity is then selected to both maintain good temporal power stability while obtaining good power output from the laser diode.
The problem with this approach is that, in optical pumping applications, the total power output from the laser diode is typically a critical design criteria. It can result in a fewer number of laser diodes being required to obtain a specified pumping power in the gain fiber. If the grating is used to provide both the power stabilizing feedback, but also the feedback to create laser operation, there is only a single variablexe2x80x94grating reflectivityxe2x80x94to optimize two parameters, total laser power output and laser power stability. In addition, very low reflectivity laser diodes ( less than 0.5%) can be hard to make, evaluate, test and screen during the manufacturing process.
As a result, another approach seeks to optimize fiber grating stabilized diode pump lasers by specifying a specific ratio between front facet reflectivity and effective grating reflectivity, taking into account the two-way coupling efficiency between the fiber pigtail and the laser diode. The problem with this approach, however, is that it fails to contemplate real-world manufacturing tolerances and specifications. To achieve acceptable yields, only ranges of front facet reflectivity, grating reflectivity, and coupling efficiency can be specified. Coupling efficiency varies with laser to fiber alignment success and culminating lens placement/alignment focusing. Fiber grating manufactures typically only provide fiber gratings having a reflectivity within a 0.5 to 1% range at reasonable costs. Finally, exit facet reflectivity can vary as much as one percent in laser diode chips manufactured from the same wafer.
As a result, according to one aspect, the invention features a fiber-grating stabilized pump module. It comprises a laser diode chip having an output facet and an optical fiber system into which light exiting from the optical facet of the laser diode is coupled. A grating is formed in the optical fiber of the optical fiber system. The grating is used to provide feedback to the laser diode to thereby promote power stability of the pump module.
According to the invention, the power reflectivity of the grating and the output facet are specified in absolute terms. Specifically, the power reflectivity of the grating is 1.0% to 2.3%, and the power reflectivity of the output facet is between 3.0 and 5.5%.
In specific embodiments, the coupling efficiency between the laser diode and the optical fiber system is between 70 and 82%.
When the optical fiber system comprises only regular fiber, with no polarization control or maintenance, a grating power reflectivity of 1.7 to 2.3% is considered to be preferable. Further, the output facet reflectivity is preferably between 3.4 and 4.9 percent.
In contrast, when the optical fiber system comprises polarization maintaining fiber, the grating reflectivity is preferably between 1.0 and 2.3%. Again, a power reflectivity of the output facet of 3.4 to 4.9% is preferable.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.